There are many ways of outlining. Here's a suggestion for those who have 'outline allergies'.
Write the first chapter (or the first two). This will get you inside the skin of the characters and will get you excited.
Now, pause and make a list of the main things you want to happen. Don't be detailed, but feel free to add a few details if they come to you.
1. car breaks down (it belonged to MC's father; add a reference to it in a later dialogue)
2. meets X at garage
3. stuff happens that allows to flesh out MC
4. meets X again 1 month later; X starts stalking MC
Go back to writing. Once you write the first time MC and X meet, go back to the outline and see if you have more things to add. If not, carry on.
But once MC meets X again, maybe you decide X didn't start stalking now, but when they first met. Go back to the outline:
- car breaks down (it belonged to MC's father; add a reference to it in a later dialogue)
- meets X at garage; X starts stalking MC
- stuff happens that allows to flesh out MC
- meets X again 1 month later because X maneuvered the situation for the meeting to happen
Now get another piece of paper (or app) and make a timeline of how X went about the stalking. If this were a visual medium (film, comics...), you could go back and insert him in the background. If it's not visual, you can go back to the 'stuff happens' and fit in a scene where the MC is going to the cinema with a friend and that friend accidentally bumps into X, who is casually described (so as to meet X's original description) without being named (since the friend doesn't know X's name).
While doing the timeline, take the chance to get inside X's plans for the future and write them down in the outline. Then check what the MC's life has in store to see if X's events will be felt.
Go back to writing.
You have to keep going back and forth, rather than just flowing on and on.
You will very likely have to insert or change scenes you've written to foreshadow the new directions and avoid plotholes.
This approach gives you flexibility, since you aren't stuck with a rigid plotline from the beginning.
You don't have to do it all in one go, so it won't feel like it's bogging you down.
Going back and forth will help you avoid plotholes and create a tight narrative.
As for tools, this approach can be done using the basics (pen and paper) or advanced (timeline apps, etc).
One thing I'm partial to is writing one chapter per event with a short description of what has to happen. If I must mention something casual in a dialogue, I'll make that note in all the chapters where it could happen. Then I write the chapters (adding or deleting chapters as the narrative progresses) and, once that mention has been inserted somewhere, I go back and erase it from the overview-notes.
I also have a detailed timeline... which gets detailed only as I write. So I may start with two events and end up with 30 main events and 101 date references to birthdays, anniversaries, start/end of projects, etc. In this timeline, I may go so far as to make an entry for the times an object or person was referred to (if it's important to keep track of it), and the timeline not only keeps track of when (chronologically) but also of where (which chapter). As I write and I decide that Y must happen, whether in the past or the future, I go back to my timeline and insert the event, making the necessary adjustments to the rest of the events. This level of detail helps avoid plotholes.
Again, take it step by step instead of trying to outline all in one go.
Not advisable if going back and forth kills your creativity. In my case, feeling the narrative coming together, adding little details for foreshadowing and adjusting scenes, boosts my confidence and my enthusiasm with the project, which means my creativity also gets a boost.